Abstract: This paper describes the design theory and implementation method of a boiler fieldbus control system. It discusses the hardware and software aspects respectively. From the actual operation results, the system improves the safety, stability and economy of the control and has broad application prospects. Keywords: Boiler control; Fieldbus control; FIX; FCS There are more than 300,000 small and medium-sized boilers in China. The annual coal consumption accounts for 1/3 of China's raw coal production. At present, most industrial and civil boilers are still in a state of low efficiency, serious environmental pollution and low safety factor [1]. In order to improve the thermal efficiency of boilers and reduce coal consumption, the use of boiler automatic control systems is becoming more and more widespread. As the control device of boiler, its main task is to ensure the safe, stable and economical operation of boilers and reduce the labor intensity of workers. Considering all factors, we use the SHCAN2000 bus control system to realize the control of boilers. The SHCAN2000 bus control system is a fieldbus control system (FCS) based on CAN bus. Since the boiler control system has high requirements for real-time performance and control reliability, the software requirements are also high. The FIX monitoring software of Intellution can meet these requirements. This article focuses on the characteristics of two 20-ton low-pressure hot water boilers in the heating system of Dalian Jinyuan Community. Based on the FCS system, it introduces a boiler fieldbus monitoring system scheme and achieves good control effect in practical application. The working principle of the whole system is as follows [2]: the standard signal of 4-20 mA or 1-5V from the control field is sent to the SHCAN2000 field control unit. After processing, the signal can be output from its AO or DO port and used to control the field equipment through the signal conditioning module; it can also be sent to the CAN bus as needed. Other field control units or host computer systems on the bus can determine whether to receive the information according to the pre-designed acceptance code and acceptance mask code. The information transmitted to the host computer can be displayed, controlled, recorded, alarmed, and printed by the host computer monitoring software. I. Hardware System of Boiler Fieldbus Control System The SHCAN2000 bus control system is different from the traditional distributed control system (DCS). It has the advanced technology and complete functions of DCS and is particularly suitable for the production process control of medium and small-sized plants. It is the result of the comprehensive utilization of computer technology, control technology and communication technology. It is a new generation of fully digital, fully distributed and fully open fieldbus control system [3]. The design concept of the control system is as follows: the signal measured by the field sensor (platinum resistance, pressure transmitter, etc.) is sent to the intelligent control unit through the signal conditioning module (temperature transmitter, input safety barrier, etc.). After being processed in the intelligent control unit, the control signal is formed. The control signal is then sent back to the field actuator (solenoid valve, shut-off valve, etc.) through the signal conditioning module (temperature transmitter, input safety barrier, etc.). Different intelligent control units can communicate with each other through the CAN bus as control centers and can send the signals to be monitored to the host computer to realize human-machine interaction. The SHCAN2000 bus control system consists of a three-layer network [4]: ​​the bottom layer is a control network connected by a low-speed fieldbus, which connects various intelligent instruments and control valves and other field devices. Each controller node is sent down to the field to form a completely distributed control system architecture; the middle layer uses a medium-speed network to connect controllers, remote I/O and intelligent I/O devices to form a system network; the top layer is the decision layer, which uses a high-speed Ethernet to connect various controllers and station-level computers. As shown in Figure 1: The bus communication standard of the SHCAN2000 system is CAN2.0B. The short frame structure, CRC check and automatic closing function of the fault node of the CAN bus ensure the reliability of signal transmission. On the operator station side, the industrial PC is connected to the CAN bus network through the network card as a data buffer; on the field control unit side, the data buffer is implemented by the standard CAN bus interface composed of SJA1000 and 82C250 chips. The CAN bus communication software enables bidirectional data exchange between the host computer and the real-time database, and between any two real-time databases. When data is exchanged between the real-time databases of any two SHCAN intelligent measurement and control components, no intervention from the host computer is required. It conforms to the ISO11898 standard and CAN2.0B technical specifications, with a maximum communication rate of 1 Mbps and a maximum communication distance of 10 km. [align=center]Figure 1. Hardware architecture of the boiler fieldbus control system[/align] The SHCAN intelligent measurement and control component is an intelligent unit that directly performs data acquisition and control functions. It can operate independently as a standalone loop regulator, or it can be placed near the field and combined with a network to form a complete data acquisition and control system. Each SHCAN intelligent measurement and control component has different built-in acquisition and control software, which can perform different function combinations, process signal acquisition, engineering quantity conversion, signal compensation, linearization processing, PID calculation, automatic alarm, fault self-diagnosis, automatic PID parameter tuning, etc., through real-time configuration and parameter settings. The "fully distributed" nature of the fieldbus control system makes the field control unit the core of the control system. The SHCAN2000 field control unit adopts an integrated technology consisting of a real-time multi-tasking operating system, real-time monitoring software, task-level configuration software, and a real-time database, supporting online configuration. II. Software Architecture of the Boiler Fieldbus Control System The software architecture of the boiler fieldbus control system consists of the following four parts, as shown in Figure 2: [align=center] Figure 2 Software Architecture of the Boiler Fieldbus Control System[/align] 1. System Monitoring Software FIX The SHCAN2000 system uses FIX DMACS from Intellution, USA, as the configuration software to complete the human-machine interface (MMI) and data operation management. The software operating platform uses Windows 98. It integrates control technology, graphics technology, database technology, and network technology, including static editing, dynamic display, alarm, historical curves, report printing, control network communication, and other functions, providing customers with a good human-machine interface and enabling real-time control. 2. SHCANI/O – I/O Driver: SHCANIO is a software module that connects the FIX DMACS to the CAN network card, and then to the SHCAN intelligent measurement and control components on the network. SHCANIO is a high-performance I/O driver developed using the I/O driver development tool (ITK) provided by Intellution. Compared with drivers implemented by conventional DDE dynamic data exchange, it has higher real-time performance and more complete functions. 3. SHCANCFG – SHCAN Download and Debugging Tool: SHCANCFG is a download and debugging tool based on Chinese Windows 95/98/NT. Through the CAN bus, it can configure, edit parameters, download, upload, manage, and monitor in real time each intelligent instrument measurement and control component on the SHCAN2000. 4. SHCAN Intelligent Measurement and Control Component Configuration Software: The SHCAN intelligent measurement and control component configuration software is a module embedded in the SHCAN intelligent instrument for configuration, data acquisition, control, input, output, and communication. III. Design and Implementation of the Boiler Fieldbus Control System The entire heating system consists of two 20t/h boilers. The hot water produced by these two boilers is collected and then sent to the heating devices of each household in the community. After heat exchange, the hot water returns to the boiler for recirculation. The combustion method is a coal-fired chain grate boiler in a firebed. It mainly consists of the boiler body, the main outlet water network, and the main return water network. In addition, there are auxiliary equipment such as induced draft fans, blowers, grate motors, circulating pumps, and dust collectors. To meet the heating needs, it is essential to ensure the stability of the boiler return water temperature and outlet water flow rate. Secondly, it is necessary to maintain the furnace pressure within a certain range to ensure production safety and combustion economy. Therefore, the controlled objects of the boiler can be determined as the boiler return water temperature, furnace negative pressure, and the outlet water pressure of the main network (which determines the outlet water flow rate). The characteristics of each of the three loops are described below: 1. From a control theory perspective, the control of a hot water boiler temperature loop is a complex object characterized by multiple variables, nonlinearity, distributed parameters, and time delay. It involves multiple controlled variables (temperature, furnace negative pressure, etc.) and regulating variables (coal quantity, forced draft, and induced draft, etc.) that interact with each other. Furthermore, there are numerous factors such as the medium, ambient temperature, and water flow rate. It is not difficult to analyze that the combustion object exhibits a large pure time delay, especially the return water temperature loop, whose time delay is even greater than the PID time constant. In a traditional PID control loop, there are three important parameters: SP (setpoint of the controlled variable), PV (actual value of the controlled variable), and OP (operational output of the PID). Generally, SP and PV are used to determine the control parameters. The deviation E of V is used to obtain the OP output through PID calculation, which in turn controls the corresponding equipment on site to adjust the temperature to approach the SP value. When the return water SP is set, due to the long lag time of the return water temperature loop, its effect takes at least 2 hours to be achieved (this mainly depends on the heating area of ​​the community). Such a large lag will inevitably lead to a large overshoot in the system, and may even cause system instability. Therefore, we control the boiler outlet water temperature. However, in order to ensure that the boiler return water temperature meets the requirements, the outlet water temperature SP value must be controlled by the return water. Here, we adopt the idea of ​​humanoid intelligent control and implement the setting of the outlet water temperature SP value through software. The implementation method is shown in Figure 3. The return water SP is generally given by the heating curve according to the heating requirements. That is, based on the outdoor temperature, the predetermined SP value for the return water temperature is given according to the curve. This value is then compared with the actual return water value PV. If the absolute value is greater than the upper deviation limit Δmax, then if the value is greater than 0, the outlet water SP value is set to the minimum SP value; otherwise, it is set to the maximum SP value. If it is not greater than the upper deviation limit Δmax, then if the value is greater than 0, the outlet water SP is set to the current value minus an offset; otherwise, the offset is added. By setting the outlet water SP in this way, not only can excessive system overshoot be avoided, but the adjustment time is also greatly shortened. In addition, since there are three factors affecting the temperature of the outlet water, namely the amount of coal added, the amount of air blown, and the speed of the grate motor, and there is a proportional relationship between the three, only by properly allocating the relationship between these three factors can the boiler operate safely, economically, and effectively. Therefore, the output OP value of the outlet water temperature PID control loop must ultimately be allocated to these three factors, namely: Coal added OP1=K1＊OP+D1 Air blown OP2=K2＊OP+D2 Grate motor OP3=K3＊OP+D3 When adjusting the six weighting coefficients K1, K2, K3, D1, D2, and D3, the economical and effective operation of the boiler can be guaranteed. [align=center] Figure 3 Outlet water SP setting block diagram[/align] 2. Furnace negative pressure loop [5] In order to ensure the effective combustion of pulverized coal in the furnace, the internal pressure of the furnace is an important parameter. Generally, the furnace negative pressure is controlled within the range of -200Pa to 200Pa. The factors affecting the furnace negative pressure are mainly the amount of air blown and the amount of induced draft. The control schemes for the two are introduced below: For the blower volume, we can use conventional PID control. However, changes in coal quantity are a major disturbance to blower control. If the blower adjustment lags behind the change in coal quantity, it will inevitably cause black smoke and seriously pollute the environment. The solution is to use the coal quantity as the feedforward quantity for blower control. This allows the blower volume to be adjusted in a timely manner when the coal quantity changes. It is worth noting that when the coal quantity increases, it is appropriate to increase the blower volume in advance. However, when the coal quantity decreases, the blower volume cannot be decreased immediately. This will also cause black smoke due to insufficient blower volume. Therefore, a judgment must be made to determine whether the coal quantity OP1 value should increase or decrease. If the coal quantity OP1 value increases, then the blower volume OP2 value should be increased immediately. If the coal quantity OP1 value decreases, then a period of time should be waited before decreasing the blower volume OP2 value. To control the furnace negative pressure, we mainly control the induced draft volume. Here, the furnace negative pressure is the controlled object, assuming a constant forced draft volume. When the forced draft volume changes, the control of the induced draft volume can also follow the forced draft control scheme. Here, the forced draft volume is used as the feedforward for induced draft control. When the forced draft OP value increases, the induced draft OP value increases immediately; however, when the forced draft OP value decreases, a waiting period is required before decreasing the induced draft OP value. 3. Water Pressure Control Loop: This loop controls the water flow rate of the community's heating network, mainly by controlling the operation of the circulating pump to change the flow rate. This is easily implemented using conventional PID control and will not be elaborated upon here. IV. Conclusion Based on the heating situation of Dalian Jinyuan Community in the winter of 2001, the fieldbus control system proposed in this paper achieved safe, economical, and efficient boiler operation, saving the community 20% of raw coal, and greatly improving the working environment of boiler operators, reducing their labor intensity, and achieving the expected goals of the heating project renovation. References: [1] Zheng Jinwu et al., Liang Daojun, Design and Research of Measurement and Control System for Low-Pressure Coal-fired Hot Water Boiler, Industrial Control Computer, Vol. 14, No. 6, 2001 [2] Yuan Aijin, Research on Software Integration Technology of Field Intelligent Measurement and Control Instruments, Journal of Instrumentation, No. 2, 2001 [3] Hao Xiaohua, Ma Xianghua, On Fieldbus Control System, Automation and Instrumentation, No. 3, 2001 [4] SHCAN2000 Distributed Control System Intelligent Measurement and Control Component System Configuration User Manual. Dalian Railway Institute Sanhe Instrument Development Company, 1999 [5] Tang Shijin, Control Technology for Energy Saving of Industrial Boilers, Ordnance Industry Press, 1993